Spasticity is a debilitating condition which affects ~75% of individuals with a spinal cord injury (SCI). Among incapacitating symptoms of spasticity is hyperreflexia which ultimately interfere with residual motor function and hamper motor recovery after SCI. Activity-based therapies are routinely integrated in SCI rehabilitation programs and result in a reduction of hyperreflexia and spasticity. The mechanisms by which exercise regulates activity in spinal pathways to reduce spasticity and improve motor function are, however, poorly understood. Persisting alterations in the action of GABA on post-synaptic targets are a signature of CNS injuries and disorders including seizure, epilepsy, Alzheimer disease, autism spectrum disorders, neuropathic pain and the development of spasticity after SCI. The inhibitory action of GABA depends on intracellular chloride concentration [Cl-]i, which is largely determined by the expression of 2 cation-chloride cotransporters (CCCs), KCC2 and NKCC1. Changing the density or activity of these transporters directly affects [Cl-]i and ultimately determines whether GABA has an inhibitory or excitatory effect. We recently found that plasticity in chloride homeostasis is involved in reflex recovery induced by exercise after SCI. Our results suggest that an increase in the expression level of KCC2 in lumbar motoneurons (MN), and subsequent decrease in [Cl-]i contributed to the restoration of spinal inhibition and reflex modulation. Plasticity in the lumbar spinal inhibitory system not only impact transmission in reflex pathways, but greatly influences walking ability after SCI. The objective of this project is to determine if the restoration of spinal inhibition through a return to chloride homeostasis is an effective target to enhance locomotor recovery after SCI. We hypothesized that after SCI 1) the decrease in KCC2/NKCC1 expression ratio in lumbar MNs contributes to the emergence of large persistent inward currents (PICs) and spasms that are recovered with step-training; 2) the increase in KCC2/NKCC1 expression ratio in lumbar MNs triggered by step-training contributes to locomotor recovery; 3) pharmacologically increasing the KCC2/NKCC1 expression ratio in lumbar MNs improves locomotor recovery and enhance the effect of step-training. The involvement of chloride homeostasis in motor impairment after SCI and recovery with step-training will be tested using KCC2 and NKCC1 pharmacological blockers, exercise and a combination of these treatments. This will lead the way for novel therapeutic perspectives for diuretics and other drugs that, in reducing the levels of [Cl-]i, reinstate the hyperpolarizing action of GABA and behaviorally relevant oscillations that are strongly dependent on GABAergic networks. This project has direct relevance to the design and implementation of future treatment and rehabilitation programs for SCI. It will define the involvement of a specific molecular pathway in both the impairment and recovery of spinal excitability and locomotion after SCI. This project will identify possible new targets to enhance pharmacological management of SCI and improve locomotor function when combined with rehabilitation programs.